Conventionally, face driving is known as a method for turning the outer circumferential surface of a material to be worked such as a shaft-shaped member. In the face driving, the outer circumferential surface of the material is turned while holding one end surface of the material in the axial direction thereof with a face driver and rotating the material with the face driver.
The face driver has a main body attached to a main spindle of a lathe or the like, a center pin protruding from the center of the main body, and a plurality of driving claws radially arranged around the center pin on the main body. The face driver holds the material by fitting the center pin into a center hole formed at the center part of the one end surface of the material and by driving the driving claws into the one end surface of the material.
The face driver integrally holds and rotates the material by making the driving claws bite into the one end surface of the material. However, if an amount of the bite of the driving claws into the one end surface of the material is small, a force of the face driver to hold the material may become less than a turning force applied to the material, which may cause a slide in the rotating direction between the face driver and the material.
In particular, if coining is performed on the one end surface of the material by cold forging or the like in order to form the center hole on the one end surface, the smoothness of the one end surface and the hardness of the material increase. As a result, the amount of the bite of the driving claws into the material becomes smaller, and the slide in the rotating direction between the face driver and the material becomes easy to occur, which may make it impossible to turn the material.
Accordingly, conventionally, as shown in JP-A 61-125703, impressions corresponding to driving claws of a face driver are formed, by pressing, on one end surface of a material to be worked, and then the driving claws are engaged with the impressions, thereby inhibiting a slide in the rotating direction between the face driver and the material.